The Function Of The Scanning Electron Microscope Biology Essay

The Scanning Electron Microscope (SEM) is one of the most powerful microscopes in the field of Science. Microscopes are used to help us examine objects that are too small to be seen in a normal human eye. Since the magnifying glass was invented, it has evolved itself through many years making today's most powerful microscope, it is also a very expensive piece of tool but it is a very useful research equipment for many applications in chemistry, medicine, molecular biology and genetics. This type of microscope allows scientist to study and observe small structural and internal cellular structures such as organelles.

The Scanning Electron Microscope creates an electronically magnified image of the sample, giving a more detailed examination because it uses a particle beam of electrons to light up the sample and make a magnified image. And throughout all existing microscopes, the SEM is more preferred being used because of it having a superior magnification than other microscopes like a light microscope, for the reason that it uses electrons that has wavelengths of around 100,000x which is shorter than photons, and it can go up to 1,000,000x magnification, while the light microscope can only go up to 1000x magnification.

This research project will show different aspects of microscopes, and how very useful they can be through many applications in modern science.

Contents:

Introduction

Aims and Objectives

Scanning Electron Microscope

Differences of Scanning Electron Microscope - Light Microscope

Systematic View of SEM - How it really works

1.4.1 Using a SEM Vacuum

1.4.2 The Electron Source

1.4.3 Sample Chamber, Lenses and its Detectors

How to prepare specimen in the SEM

Application of Scanning Electron Microscope into Forensics

2.1. Scanning Electron Microscope in Forensic Science

2.2. Criminal Case Study

2.2.1 Ballistics Investigation - Scanning Electron Microscope

2.2.2 Paint Chip Investigation - Scanning Electron Microscope

3. About this Project

3.1. Discussion/Conclusion

3.2. References

1.) Introduction

1.1.

The aim of this project is to understand how the (SEM) - Scanning Electron Microscope works throughout many applications.

* Objectives for this project:

- Thoroughly explain (SEM)

- History of (SEM)

- Identify the Pros and Cons

- How (SEM) helps Forensics Science

1.2. Scanning Electron Microscope

For this research project, I've chosen SEM, which stands for Scanning Electron Microscope, because of its unique ways of giving a very defined image of a sample, for the reason that this microscope uses electrons instead of light to form an image. Since the advancement of SEM in the early 1950's, the scanning electron microscope had developed contemporary areas of study in the medical and physical field in science. It has allowed research scientist to study many different type of samples.

Over other traditional microscopes, the SEM has a bigger depth of field that allows more samples to be seen at once. It also has a very high resolution, so if samples are close it can be magnified at much higher levels because scanning electron microscope has a built it electromagnets rather than using lenses that of other microscopes use, this gives more control for the user to manipulate the magnification, the magnification of SEM is greater than 100,000x.

The scanning electron microscope produces a beam of incident electrons in an electron column above the specimen chamber. Once the beam pass through the electromagnets and lenses, which focus the beam down the specimen, and the beam reaches the specimen, electrons and x-rays are then ejected from the specimen as it shows in the picture 2 below.

Picture 2

The two detectors (backscattered electron detector and secondary electron detector) will then collect the x-rays, backscattered electrons and secondary electrons and alter them into a signal that is sent to a screen (monitor screen), which makes the final image of the specimen.

The incident electrons cause electrons to be released from the specimen due to elastic and inelastic spreading events inside the specimen's exterior. The high energy electrons that got ejected by an elastic collision of an incident electron, usually with a specimen atoms nucleus are referred to as the backscattered electrons. To make the specimen's appear brighter, it varies the samples atomic number. So if the samples atomic number is high then the brighter it appears than lower atomic number specimens.

To make an image, the incident electron beam is scanned in a computer graphics and imaging, a set of pixels orderly in rows and columns, which is to form an image of the specimen's exterior. The electrons that had been released are detected for each position in the scanned area by the electron detector. The brightness displayed will be determined by the intensity of the released electron on a (CRT) Cathode Ray Tube, by levelling up the CRT scan to that of the scan of the incident electron beam, the CRT display will represent the classification, analysis and the picture of the formation of the specimen's surface area.

Between the two microscopes Scanning Electron Microscope and Light Microscope don't have much in common except both can be used to look at specimens on a larger scale. Both electron and light microscopes are technical devices that are used for making images of a specimen's structure that are too small to see with the help of magnification, and both types have relevant areas of applications in biology and the materials sciences which will be in a more deep explanation in this research project.

Mainly both the scanning electron microscope and the light or optic microscope functions by bouncing tiny particles or waves off of matters in order to make them visible. However, electrons are smaller than light photons so we can see smaller objects clearly with the electron microscopes. And also there are disadvantages. First, colour is a function of light wave frequency so this mean non can see electron micrographs in colour. However colour can be added artificially using a computer to tell the differences between structures. Also, non can see electrons so it makes them visible by bouncing them off an cathode ray screen, or a computer monitor.

There are also other known disadvantages of SEM, one was mentioned above which is the lack of ability to replicate colour, this is because electrons have none, which only makes a greyscale pictures of the specimens on the monitor as shown below (picture 1). Also a sample must be stable under vacuum, and in most cases the sample must be electrically conductive because conductive thin film coatings can reduce vacuum and also conductivity, which may create problems throughout the procedure.

Picture 1

The electron microscope consist of a cylindrical tube of about 2 metres long which is totally devoid of air, and in differences to the light microscope, the SEM pictures matters via a thin beam of fast moving electrons that interfere with the sample in the tube. The electrons are then emitted by the cathode on top of the tube, and then accelerated by the anode -as you can see on the systematic view of SEM above-, they then pass through a small aperture, which forms them into a beam, and into the vacuum inside the tube.

The part of the microscope that produces the electron beam is called the electron gun; the beam is then preserved along the tube by means of the electromagnetic lenses, this are coils that surrounds the tube at given gaps, the electromagnetic field emitted by the coils focuses the beam at the centre of the tube, by the electrons encounter the sample and that either absorbed, scattered or pass through it. Because different regions of the sample are variously transparent to electrons different amounts of electrons with changed energy pass through this regions, at the end of the tube the electrons are collected on the screen to make an image of the sample. The beam that reaches the film consists of the different amounts of electrons that pass through particular regions of the sample, this difference is responsible for the contrast in the film, the original image produce by the electron microscope is always greyscale and it is not possible to see it directly with the eye.

1.4.1 Using a SEM - Vacuum

www.reading.ac.uk/cfam/facilities/coater3.jpg

When using the SEM, the column should constantly be at a vacuum. This is because if the specimen is surrounded with gas, an electron beam wont be produced for the reason that of a high instability in the beam, and for the reason that gases could react with the electron source which causes the electrons in the beam to ionise and make accidental discharges and might lead to instability in the beam.

Also the transmission of the beam throughout the electron optic column may also be slowed down by the existence of molecules that may have come from the specimen or the microscope itself that might form compounds and condense on the specimen. If this happens, it may lower the contrast and making the produced image unclear.

1.4.2 Electron Source

http://mse.iastate.edu/images/microscopy/electron_gun.gif

Most type of filaments used on an SEM is a tungsten hairpin gun, which is a filament that has a loop of tungsten that works as the cathode. The electron beam comes from a filament through many different types of materials. A voltage is applied on the loop that causes it to heat up dramatically. The anode then forms a great attractive force for electrons, which then causes them to accelerate towards the anode or the specimen.

1.4.3 Specimen Chamber, Lenses and its Detectors

The specimen chamber is positioned down the column of the SEM, as the it shows in the picture below.

http://www.mse.iastate.edu/images/microscopy/sample_chamber.jpg

And inside the specimen chamber, the lenses and the detectors are placed inside as it shown below.

http://www.mse.iastate.edu/images/microscopy/detectors3_tag.jpg

* The EDS or Energy Dispersive X-ray Spectroscopy changes the energy of each single x-ray into a voltage signal of relative size. This is done all through three stages:

The x-ray is changed into a charge by the ionisation of atoms inside the semiconductor crystal.

Then the converted x-ray into charge becomes a voltage signal by the FET - Field Effect Transistor.

Then lastly the voltage signal is inputted into the pulse processor for measurements.

* The secondary detector detects the low powered secondary electrons that are emitted from the k-orbitals of the samples atom by inelastic scattering exchanges with the beam electrons.

* The third detector is the backscatter detector, which detects the backscattered electrons that consist of high powered electrons that comes from the electron beam and is reflected out of the sample by elastic scattering with the samples atom.

The Objective Lenses

http://www.mse.iastate.edu/images/microscopy/lens.jpg

The objective lenses are the one that focuses the electron beams towards the specimen as it shows the lens above.

SEM Stage

http://www.mse.iastate.edu/images/microscopy/stage1.jpg

As you can see above the picture is where the prepared specimen is put on called the stage SEM.

1.5. Preparing Specimens in the SEM

http://www.mos.org/sln/SEM/sem_9.gif

Before examining a specimen for SEM, a specimen must be coated with a metal usually used is gold from which the received electrons will bounce back. The repelled electrons are then detected and changed into a optical illustration

2.) Application of Scanning Electron Microscope to Forensic Science

2.1. Scanning Electron Microscope in Forensic Science

Not only the Scanning Electron Microscope is important to other Science applications, it is also a vital instrument in Forensic Science due to its broad variety of uses. This is because; it allows fast study of elements that has very small sample and the final determination of the origin of materials that are essential to the series of proof such as gunshot remains, bullet fingerprints, counterfeit bank notes, fibres, paint particles and forged documents, etc are all examples of samples that can be examined using the this tool - Scanning Electron Microscope. Also as said before, the SEM makes detailed 3D pictures of very small things - microorganisms, organic structures or any examination of gems or fragments.

The aim of using scanning electron microscope in forensic investigative tests is to construct individualized data that allows the identification and the source of the specimen, also to precisely interpret the data in connection to the crime or to the suspect under investigation or to assist and give details of an explosion, arson or a accidental plane crash - or non accidental.

From the progress of the scanning electron microscopy in forensics, it gives non-negative testing of both natural and non-living specimens. Also a different application of this technique in forensics is the examination and identification of dust and particles in the air of indoor surroundings to either evaluate the air quality or to identify potential pathogens, that are highly dangerous because they are disease causing organisms. Also all these materials that have been used in forensic investigation done with SEM are: - Mineral grains like glass, mica and carbonates - Biological materials like pathogen spores, insect particles, skin cells and rodent - Fibers like textile fibers, carpet fibers, hair and cellulose. All these can be investigated using the scanning electron microscope; this is why the SEM is one of the most useful equipment to use in forensics.

2.2. Criminal Case Study

For my case study it is an unpopular criminal case. It involves a shooting in Manchester, UK, which was a murder inquiry of a 16 yrs old boy shot dead outside a pub car park as he was been chased and shot; it was reported as the first serious shooting in the city for a year of related gang shooting. The victims name is Giuseppe Gregory, who grew up and lived with his mother in a town called Ardwick.

Giuseppe Gregory was shot in the head by a group of gang members who was driving a green coloured VW Golf in the car park of a pub known as the Robin Hood pub, the car had crashed to another vehicle. The police believe that the group of gunman had taken the same vehicle fleeing the scene of the crime, the victim who was rushed in to a hospital died shortly at Trafford general hospital. Afterwards three men were arrested on suspicion of murder aged 18, 20 and 22.

The use of the scanning electron microscope is very vital at this point, because it can be used to compare the bullet recovered from the victim - deceased and from the loaded gun that was used.

2.2.1 Ballistics Investigation - Scanning Electron Microscope

In ballistics investigation, the firing pins, extractors and breech of marks all leave a mark or also known as mechanical fingerprints on the bullets and cartridge cases passing through the case hardened action of a gun. A forensic investigator can use the scanning electron microscope to locate, examine and identify the class of bullet or cartridge case and compare these marks using the powerful magnification of the SEM.

And also the scanning electron microscope allows the study of materials in micro and submicron ranges. It is also capable of generating 3D pictures for examination of as it shows on the pictures above; the analyst can also perform an element testing to detect contaminants that are possibly present.

This can help the police investigation by comparing the bullets found and collected from the suspects and used on the victim. It can also add to the evidence presented in the court and help the jury make the right decision.

Also gun residue can also be identified using the scanning electron microscope based on the combination of unique size, shape and chemical composition.

And with the scanning electron microscope, the gun residue can be analyzed, counted and sorted in a totally automated method that saves time for the analyst.

2.2.2 Paint Chip Investigation - Scanning Electron Microscope

Paint chip can also be at help with an investigation as provided and used as evidence, this is because paint has descriptions such as colour, texture, thickness and chemical composition of the paint layers that scanning electron microscope imaging exposes as it shows below.

It reveals information about the paints layers, thickness and the composition of it.

Also in addition to imaging the layers for comparison, the forensic analyst can use the EDS - Energy Dispersive X-ray Spectroscopy to examine the chemical composition.

3.) About this Project

This project was about thoroughly explaining and learning about the SEM - Scanning Electron Microscope of its uses and how very useful it is to Forensic Science. Even though the SEM has a few disadvantages it still remains the most vital tool for every modern science and the most preferred microscope out of all.

3.1. Discussion/Conclusion

I think it is really vital to have the right tools available to get the job done precisely and accurately on time, and also I think it made the job of a Forensic analyst more easier.

Throughout my research I have gained the knowledge of how SEM - Scanning Electron Microscope really works and how it really helps Forensic Science. I have learned that the SEM is not only useful for normal everyday analysis of things such as looking at materials and such; it can also help solve crime by analysing materials that are found in the scene of the crime like clothing fibres, gun cartridges, gun residue and paint chip and it can even reconstruction an accident.

I have learned even accidents can be determined if it was or not by using the Scanning Electron Microscope. It involves a vehicle's headlights, an SEM image of the headlight filament can tell if it was an accident or not. If the cars headlights were on or off when the accident happened. The indication that the headlights were off is when a sharp break in the filament. And if the indication that the headlights were on is when the filament is melted.

This is why the Scanning Electron Microscope is a very important tool for Forensic Science as it helps greatly with criminal, accidents and non-crime related investigations. Although I think it lacks a few things that may make a little difference and make SEM better, such as the colour of images it produces.

In conclusion, even though the SEM - Scanning Electron Microscope is very expensive, it is worth it because of its usefulness and the easy use of the device and along with the capabilities that this piece of tool can do.